Power amplifier circuits are used in transmitters for radio communication in order to amplify a signal and transmit radio waves into the air.
A switched mode power amplifier circuit is known as a type of power amplifier circuit. Furthermore, a polar modulation-based switched mode power amplifier circuit is known as a type of switched mode power amplifier circuit. With a polar modulation-based switched mode power amplifier circuit, a modulated wave signal is separated into a phase signal and an amplitude signal. A power amplifier is driven by the phase signal and, for example, the number of unit amplifiers to be operated of a plurality of unit amplifiers is controlled by the amplitude signal. Because a switched mode power amplifier circuit operates as a switch, it is hardly subject to variations in the characteristics of elements. Furthermore, a switched mode power amplifier circuit operates stably even at a low power supply voltage. Therefore, a switched mode power amplifier circuit is suitable for a semiconductor integrated circuit fabricated by microfabrication processes.
See, for example, International Publication Pamphlet No. WO2009/151097.
By the way, with a power amplifier circuit in which the number of unit amplifiers to be operated of a plurality of unit amplifiers is controlled by an amplitude signal, the number of unit amplifiers to be controlled may be reduced by making a digital code of the amplitude signal small to decrease average output power. In that case, however, the signal to noise ratio (SNR) of the amplitude signal becomes low and the SNR of output power of the power amplifier circuit also becomes low. This makes it difficult to exercise power control with accuracy in a wide power range.